Polyethylene films with improved physical properties

ABSTRACT

Metallocene catalyzed polyethylenes are found to have improved physical properties, improved processability and improved balance of properties. Surprisingly, there is a direct relationship between MD shrinkage, and MD tear. Additionally, MD tear is greater than TD tear, and MD tear is also greater than intrinsic tear. MD tear to TD tear ratios are above 0.9, and dart drop impact is above 500 g/mil. The polyethylenes have a relatively broad composition distribution and relatively broad molecular weight distribution.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a Continuation Application of U.S.Ser. No. 10/199,446, filed Jul. 19, 2002, which claims priority toProvisional Application U.S. Ser. No. 60/306,600 filed Jul. 19, 2001.

TECHNICAL FIELD

[0002] The present invention relates to films that exhibit a superiorbalance of physical properties, and a metallocene catalyzed polyethyleneused to make the films that is easier to process than previousmetallocene catalyst produced polyolefins and/or polyethylenes. Thefilms are produced with polyethylenes having a relatively broadcomposition distribution (CD) and a relatively broad molecular weightdistribution (MWD).

BACKGROUND

[0003] Metallocene-catalyzed ethylene inter-polymers are known, whichhave improved processing and film properties, such as dart drop impactstrength (dart). However, none of the existing body of knowledgeachieves the balance of physical properties, the molecular propertiesand ease of processing discussed herein. Conventional Ziegler-Nattacatalyzed linear low density polyethylene (Z-N LLDPE) is known to havegood stiffness, as expressed by 1% secant modulus, and good Elmendorftear strength.

[0004] However, conventional knowledge in the polyethylene film art isthat by increasing the machine direction orientation (MD) in filmsduring manufacture of these films, physical properties, such as MD tearstrength, will decrease.

[0005] To this point, in Polymer Engineering and Science, mid-October1994, vol. 34, No. 19, the disclosure of which is incorporated herein byreference, the authors discuss processing structure propertiesrelationships in polyethylene blown film. The authors suggest that MDElmendorf tear is found to be inversely related to drawdown ratio and MDshrinkage.

[0006] Further, in Polymer, 41 (2000) 9205-9217, the disclosure of whichis incorporated herein by reference, the authors suggest that at high MDextension rates, a greater number of molecules will be oriented alongthe MD prior to the onset of crystallization, and that this isdetrimental from a MD tear performance perspective.

[0007] Metallocene catalyst components can be combined to form blendcompositions as described in PCT publication WO 90/03414 published Apr.5, 1990, the disclosure of which is incorporated herein by reference.Also mixed metallocenes as described in U.S. Pat. Nos. 4,937,299 and4,935,474, the disclosure of both which are incorporated herein byreference, can be used to produce polymers having a broad molecularweight distribution and/or a multimodal molecular weight distribution.

[0008] U.S. Pat. No. 5,514,455 suggests that a reduction in gauge ofpolyethylene films results in an increase in tear values. This documentemploys a titanium magnesium catalyst for polyethylene production andincludes titanium residues in the polyethylene. Reported values ofElmendorf machine direction (MD) tear to transverse direction (TD) tear,are in the range of 0.1-0.3 for inventive examples.

[0009] U.S. Pat. No. 5,744,551, the disclosure of which is incorporatedherein by reference, suggests a balance of tear property improvement.This document also employs a titanium magnesium catalyst forpolyethylene production and includes titanium residues in thepolyethylene. Further, the MD/TD tear ratios are in the range of0.63-0.80 for inventive examples.

[0010] U.S. Pat. No. 5,382,630, the disclosure of which is incorporatedherein by reference, discloses linear ethylene interpolymer blends madefrom components that can have the same molecular weight but differentcomonomer contents, or the same comonomer contents but differentmolecular weights, or comonomer contents which increase with molecularweight. U.S. Pat. No. 5,382,630 suggests multimodal polyethylene blendsfor which tear strength can be controlled. However, this document usesonly intrinsic tear, and is silent on Elmendorf MD/TD tear ratios and onany other values but intrinsic tear.

[0011] Also, in U.S. Pat. No. 6,242,545 and U.S. Pat. No. 6,248,845 aswell as provisional applications U.S. Ser. Nos. 60/306,503 filed Jul.19, 2001 and 60/306,903 filed Jul. 19, 2001,, the disclosures of allwhich are incorporated herein by reference, the patentees/applicants ofthese documents reported production of either broad compositiondistribution, narrow molecular weight, or broad compositiondistribution, relatively broad molecular weight distributionpolyethylenes. However, these documents show an improvement in cast filmMD tear, but no appreciable improvement for blown film.

[0012] There is a commercial need therefore for a polyolefin film, morespecifically a blown polyethylene film, that has high machine directiontear (MD tear) and/or high transverse direction tear (TD tear), and/orhigh dart drop impact resistance (dart), made from a polyethylene thatis easier to process than prior metallocene catalyst produced linear lowdensity polyethylenes (mLLDPE). In other words it is desirable to havethe processability, stiffness and tear strength of a ZN-LLDPE combinedwith the dart impact strength of a mLLDPE.

SUMMARY

[0013] Surprisingly, we have now discovered that films exhibiting suchimproved physical properties are the result of a polyethylene that isproduced in a single reactor, with a substantially single catalyst. Suchimproved physical properties are unexpected and surprising. Furthermore,the MD tear strength of these films can be increased by increasing theMD orientation of these films during their manufacture, which is alsounexpected and surprising.

[0014] We contemplate a film, comprising a linear low densitypolyethylene (LLDPE), wherein the film has a ratio of MD tear to TDtear, both determined by ASTM D 1922, of ≧0.9, or ≧1.0, or ≧1.1, or≧1.2, or ≧1.3.

[0015] In another embodiment we contemplate that such films will have anMD tear ≧350 g/mil, and a dart drop impact, as determined by ASTMD-1709≧500 g/mil. We further contemplate a process for producing suchfilms by extruding an LLDPE at a temperature effective to produce a filmhaving an MD tear≧350 g/mil.

[0016] Also contemplated is a process for producing these films byextruding the film of an LLDPE at a drawdown ratio effective to producea film having an MD tear≧350 g/mil.

[0017] Additional embodiments include: a polyolefin film, comprising anLLDPE extruded at a temperature, or a drawdown ratio effective to form afilm having a MD tear to TD tear ratio≧1.0, wherein the film has a MDtear≧450 g/mil and a dart drop impact≧500 g/mil. Contemplated as well isa film having a balance of physical properties, comprising an LLDPE,wherein the film has a ratio of MD tear to TD tear, as measured by ASTMD 1922, of ≧1.0, and a MD tear≧400 g/mil, and a dart drop impact, asmeasured by ASTM D-1709≧500 g/mil. Further, we contemplate apolyethylene film, comprising: an LLDPE, the film having an MD tear≧500g/mil, and an MD shrinkage≧70%.

[0018] Also contemplated is a polyethylene film, comprising an LLDPE,wherein the film has a direct relationship between MD tear and MDshrinkage or draw-down ratio.

[0019] We further contemplate a LLDPE having a molecular weightdistribution, in the range of from 2.5-7, and a broad, polymodalcomposition distribution, the composition distribution breadth index(CDBI) generally ≦55%, as determined by crystallization analysisfractionation (CRYSTAF).

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] These and other features, aspects and advantages of embodimentsof our invention, will become better understood with reference tofollowing description, appended claims, and the accompanying drawing, inwhich:

[0021]FIG. 1 shows data plotted as MD tear versus MD shrinkage. The filmdata from embodiments of the present invention are plotted as well asthe data from Polymer Engineering and Science, mid-October 1994, vol.34, No. 19 for comparison.

DETAILED DESCRIPTION

[0022] In certain embodiments of our invention, films having a uniquebalance of MD and TD tear, and/or a simultaneously increasing MD tearwith increasing MD shrinkage are contemplated.

[0023] Following is a detailed description of certain combinations ofpolyethylene manufacturing techniques, the use of such polyethylenes somade into films, and the fabrication of these films into useful articlessuch as trash bags or heavy duty shipping sacks, and use of thesearticles. Those skilled in the art will appreciate that numerousmodifications to these embodiments may be made without departing fromthe scope of our invention. For example, while certain specific filmextrusion techniques using certain polyethylenes are discussed andexemplified, other polyethylenes and extrusion parameters are alsocontemplated. Additionally, while trash bags and heavy duty shippingsacks are among the uses for embodiments of our inventive films, otheruses are also contemplated.

[0024] To the extent that this description is specific, it is solely forthe purpose of illustrating certain embodiments of the invention andshould not be taken as limiting the present inventive concepts to thesespecific embodiments.

[0025] The film of the invention may have a total thickness ranging from≧0.1, or ≧0.2, or ≧0.3 mils, (≧2.5, or ≧5.1, or ≧7.6 microns) or ≦3 or≦2.5, or ≦2, or ≦1.5, or ≦1, or ≦0.8, or ≦0.75, or ≦0.6 mils (≦76 or≦64, or ≦51, or ≦38, or ≦25, or ≦20, or ≦19, or ≦15 microns.

[0026] Catalyst Components and Catalyst Systems

[0027] Embodiments of our invention include use of a hafnium transitionmetal metallocene-type catalyst system as described in U.S. Pat. No.6,242,545 and/or U.S. Pat. No. 6,248,845, hereby incorporated byreference. The techniques for catalyst preparation are included in thesedocuments and the techniques are also exemplified by Example 1 herein.

[0028] Additionally, in another embodiment, the method of the inventionuses a polymerization catalyst in a supported form, for exampledeposited on, bonded to, contacted with, or incorporated within,adsorbed or absorbed in, or on, a support or carrier. In anotherembodiment, the metallocene is introduced onto a support by slurrying apresupported activator in oil, a hydrocarbon such as pentane, solvent,or non-solvent, then adding the metallocene as a solid while stirring.The metallocene may be finely divided solids. Although the metalloceneis typically of very low solubility in the diluting medium, it is foundto distribute onto the support and be active for polymerization. Verylow solubilizing media such as mineral oil (e.g. Kaydo® or Drakol®) orpentane may be used. The diluent can be filtered off and the remainingsolid shows polymerization capability much as would be expected if thecatalyst had been prepared by traditional methods such as contacting thecatalyst with methylalumoxane in toluene, contacting with the support,followed by removal of the solvent. If the diluent is volatile, such aspentane, it may be removed under vacuum or by nitrogen purge to affordan active catalyst. The mixing time may be greater than 4 hours, butshorter times are suitable. Such techniques are also exemplified byExample 23 herein.

[0029] Polymerization Process of the Invention

[0030] The substituted bulky ligand hafnium transition metalmetallocene-type catalyst compounds and catalyst systems discussed aboveare suited for the polymerization of monomers, and optionally one ormore comonomers, in any polymerization process, solution phase, gasphase or slurry phase.

[0031] In an embodiment, our invention is directed toward the solution,slurry or gas phase polymerization or copolymerization reactionsinvolving the polymerization of one or more of the monomers having from2 to 30 carbon atoms, or 2-12 carbon atoms, or 2 to 8 carbon atoms. Theinvention is well suited to the copolymerization reactions involving thepolymerization of one or more of the monomers, for example alpha-olefinmonomers of ethylene, propylene, butene-1, pentene-1,4-methyl-pentene-1, hexene-1, octene-1, decene-1, and cyclic olefinssuch as cyclopentene, and styrene or a combination thereof. Othermonomers can include polar vinyl monomers, diolefins such as dienes,polyenes, norbomene, norbornadiene, acetylene and aldehyde monomers.Generally a copolymer of ethylene is produced.

[0032] In another embodiment, the process of the invention relates tothe polymerization of ethylene and at least one comonomer having from 4to 8 carbon atoms. The comonomers may be butene-1,4-methyl-1-pentene;hexene-1 and octene-1.

[0033] Typically in a gas phase polymerization process a continuouscycle is employed where in one part of the cycle of a reactor, a cyclinggas stream, otherwise known as a recycle stream or fluidizing medium, isheated in the reactor by the heat of polymerization. This heat isremoved in another part of the cycle by a cooling system external to thereactor. (See for example U.S. Pat. Nos. 4,543,399, 4,588,790,5,028,670, 5,317,036, 5,352,749, 5,405,922, 5,436,304, 5,453,471,5,462,999, 5,616,661 and 5,668,228 all of which are fully incorporatedherein by reference.)

[0034] Generally, in a gas fluidized bed process for producing polymers,a gaseous stream containing one or more monomers is continuously cycledthrough a fluidized bed in the presence of a catalyst under reactiveconditions. The gaseous stream is withdrawn from the fluidized bed andrecycled back into the reactor. Simultaneously, polymer product iswithdrawn from the reactor and fresh monomer is added to replace thepolymerized monomer. The reactor pressure may vary from 100 psig (680kpag)-500 psig (3448 kPag), or in the range of from 200 psig (1379kpag)-400 psig (2759 kpag), or in the range of from 250 psig (1724kpag)-350 psig (2414 kPag). The reactor temperature may vary between 60°C.-120° C., or 60° C.-115° C., or in the range of from 70° C.-110° C.,or in the range of from 70° C.-95° C., or 70-90° C. The productivity ofthe catalyst or catalyst system is influenced by the main monomerpartial pressure. The mole percent of the main monomer, ethylene, isfrom 25-90 mole percent, or 50-90 mole percent, or 70-85 mole percent,and the monomer partial pressure is in the range of from 75 psia (517kPa)-300 psia (2069 kPa), or 100-275 psia (689-1894 kPa), or 150-265psia (1034-1826 kPa), or 200-250 psia (1378-1722 kPa), which are typicalconditions in a gas phase polymerization process.

[0035] The settled bulk density for the polymers produced by the processof invention are in the range of from 10-35 lb/ft³ (160-561 kg/m³), orfrom 12-35 lb/ft³ (193-561 kg/m³), or from 14-32 lb/ft³ (224-513 kg/m³),or from 15-30 lb/ft³ (240-481 kg/m³).

[0036] Other gas phase processes contemplated by the process of theinvention include those described in U.S. Pat. Nos. 5,627,242, 5,665,818and 5,677,375, and European publications EP-A-0 794 200, EP-A-0 802 202and EP-B-634 421 all of which are herein fully incorporated byreference.

[0037] One process embodiment of the invention is a process, a slurry orgas phase process, or a gas phase process, operated in the substantialabsence of or essentially free of any scavengers, such astriethylaluminum, trimethylaluminum, tri-isobutylaluminum andtri-n-hexylaluminum and diethyl aluminum chloride and the like. Thisprocess is described in PCT publication WO 96/08520, which is hereinfully incorporated by reference.

[0038] A slurry polymerization process generally uses pressures in therange of 1 to 50 atmospheres and even greater and temperatures in therange of 0° C. to 200° C. In a slurry polymerization, a suspension ofsolid, particulate polymer is formed in a liquid polymerization mediumto which ethylene and comonomers and often hydrogen along with catalystare added. The liquid employed in the polymerization medium can bealkane or cycloalkane, or an aromatic hydrocarbon such as toluene,ethylbenzene or xylene. The medium employed should be liquid under theconditions of polymerization and relatively inert. Hexane or isobutanemedium may be employed.

[0039] In one embodiment a polymerization technique of the invention isreferred to as a particle form, or slurry process where the temperatureis kept below the temperature at which the polymer goes into solution.Such technique is well known in the art, see for instance U.S. Pat. No.3,248,179 which is fully incorporated herein by reference. Thetemperature in the particle form process is within the range of 185° F.(85° C.) to 230° F. (110° C.). Two polymerization methods for the slurryprocess are those employing a loop reactor and those utilizing aplurality of stirred reactors in series, parallel, or combinationsthereof. Non-limiting examples of slurry processes include continuousloop or stirred tank processes. Also, other examples of slurry processesare described in U.S. Pat. No. 4,613,484, which is herein fullyincorporated by reference.

[0040] In one embodiment the reactor utilized in the present inventionis capable of producing greater than 500 lbs/hr (227 Kg/hr) to 200,000lbs/hr (90,900 Kg/hr)or higher of polymer, or greater than 1000 lbs/hr(455 Kg/hr), or greater than 10,000 lbs/hr (4540 Kg/hr), or greater than25,000 lbs/hr (11,300 Kg/hr), or greater than 35,000 lbs/hr (15,900Kg/hr), or greater than 50,000 lbs/hr (22,700 Kg/hr) or greater than65,000 lbs/hr (29,000 Kg/hr) to greater than 100,000 lbs/hr (45,500Kg/hr).

[0041] Polymer Product

[0042] The polymers used by this invention typically have a density inthe range of from 0.86 g/cc-0.97 g/cc, or in the range of from 0.88g/cc-0.965 g/cc, or in the range of from 0.900 g/cc-0.96 g/cc, or in therange of from 0.905 g/cc-0.95 g/cc, or in the range from 0.910g/cc-0.940 g/cc, or greater than 0.910 g/cc, or greater than 0.915 g/cc.Polyethylenes in 0.915-0.940 g/cc generally are considered LLDPE. Thepolymers of the invention typically have a molecular weightdistribution, a weight average molecular weight to number averagemolecular weight (M_(w)/M_(n)) of 2.5-7, or 2.7-6, or 2.8-5 Also, thepolymers of the invention typically have a broad, polymodal compositiondistribution (CDBI), generally ≦55%, or≦50%, or≦45%, or≦40% asdetermined by CRYSTAF. In another embodiment, the polymers produced bythe process of the invention, particularly in a slurry and gas phaseprocess, contain less than 5 ppm hafnium, generally less than 2 ppmhafnium, or less than 1.5 ppm hafnium, or less than 1 ppm hafnium.Additionally, we contemplate that polyethylenes of embodiments of ourinvention will have small to no amounts of titanium, ≦5 ppm, or ≦3 ppm,or ≦1 ppm, or zero detectable by current analytical techniques.

[0043] Polymers used to make the film of the invention are also usefulin other forming operations such as sheet, and fiber extrusion andco-extrusion as well as blow molding, injection molding and rotarymolding. Films include blown or cast films formed by coextrusion or bylamination useful as shrink film, cling film, stretch film, sealingfilms, oriented films, snack packaging, heavy duty bags, grocery sacks,baked and frozen food packaging, medical packaging, industrial liners,membranes, etc. in food-contact and non-food contact applications.Fibers include melt spinning, solution spinning and melt blown fiberoperations for use in woven or non-woven form to make filters, diaperfabrics, medical garments, geotextiles, etc. Extruded articles includemedical tubing, wire and cable coatings, geomembranes, and pond liners.Molded articles include single and multi-layered constructions in theform of bottles, tanks, large hollow articles, rigid food containers andtoys, etc.

[0044] In one embodiment of this invention, the polymerization productis a linear low-density polyethylene (LLDPE) resin produced bypolymerization of ethylene and an alpha-olefin comonomer, or hexene-1,or octene-1, to the extent of an incorporated content of alpha-olefincomonomer of from 1 to 5 mole %, for a polymer density of ≧0.915 g/ccand a polymer weight average molecular weight ≧25,000 and such ethylenealpha-olefin copolymer is produced with a catalyst system the hafnocenecomponent of which is at least 95 mole %, or greater of all transitionmetal compound component. In another embodiment of this invention, theLLDPE resins so produced by a catalyst system of this invention isthereafter converted into a film article.

[0045] Film Extrusion and Film Properties

[0046] The LLDPE resins used in the invention are more easily extrudedinto film products by cast or blown film processing techniques withlower motor load, higher throughput and/or reduced head pressure ascompared to EXCEED™ resins (available from ExxonMobil Chemical Co.) ofcomparable melt index, comonomer type and density. Such LLDPE resinshave, for a comparable MI, a higher weight average molecular weight anda broader MWD than does an EXCEED™ resin. The improvement in the balanceof tear properties can be expressed as a ratio of MD to TD tear(Elmendorf). For embodiments of our invention, this ratio will generallybe ≧0.9, or ≧1.0, or ≧1.1, or ≧1.2, or ≧1.3. In another embodiment, MDtear values of ≧350 g/mil, or ≧400 g/mil, or ≧450 g/mil or ≧500 g/milare contemplated. Intrinsic tear, determined by using the same test asboth MD and TD tear, but prepared by compression molding a plaque, isgenerally believed to be greater than MD tear for LLDPE materials.However, in embodiments of our invention, the intrinsic tear divided byElmendorf MD tear will be ≧1, or ≧1.1, or ≧1.2, or ≧1.4, or ≧1.6. Inother embodiments, the dart drop impact resistance (dart) will be ≧500g/mil (≧500 g/0.254 mm), in other embodiments this dart will be combinedwith the excellent MD tear value. In other embodiments, processingparameters used during the manufacture of film can be used to improvethe physical properties, especially MD tear. It is known that parameterssuch as melt temperature (extrusion), die gap, blow-up ratio (BUR), andfinal film gauge, can influence film properties. Draw down ratio (DDR)is defined as:${DDR} = \frac{{die}\quad {gap}}{{final}\quad {film}\quad {thickness} \times {BUR}}$

[0047] The residual stresses put into the film as a result of theseextrusion variables, can be correlated with shrinkage measurements.Typically, there is a direct relationship between shrinkage and DDR, andan inverse relationship between shrinkage and MD tear. In embodiments ofour invention, we find a direct relationship between shrinkage and DDR,but contrary to prior art and unexpectedly and surprisingly, we find adirect relationship between MD tear and MD shrinkage.

[0048] In such embodiments, the MD tear will be ≧500 g/mil, or ≧550g/mil and concurrently, the MD shrinkage will be ≧70%, or ≧75%.

[0049] Additionally, while some physical property improvements may beseen on thick film, in excess of 3 mils, using other polyethylenes andformed without using the film extrusion techniques outlined herein, wegenerally contemplate that commercial films, utilizing the presentlydisclosed polyethylenes and extrusion techniques, and articles madetherefrom, will have a thickness of ≦3 mils, or ≦2 mils or ≦1 mil, or≧0.1 mil or ≧0.2 mils. Property improvements can also be seen at low,generally unacceptable, non-commercial extrusion rates. The filmproperties of embodiments of our invention however, will generally beattainable at ≧8, or ≧10 or ≧12 or ≧14 , or ≧16, or ≧18 or more poundsof polymer output, per hour, per inch of die circumference.

[0050] LLDPEs as described above, with hexene as comonomer will exhibita weight average molecular weight of 25,000-200,000 at corresponding MIvalues that range between 10-0.1 MI, and the weight average molecularweight ranges from 80,000-150,000 within which range the melt indexrespectively ranges from a value of 3-0.5. For such LLDPE resins, themelt index ratio (MIR defined by I₂₁/I₂ described herein) is ≧20 or ≦40,and or ≧25 or ≦35.

[0051] Typical die gaps range from 30-120 mils, or 60-110 mils. Melttemperatures range from 350-550° F., or 390-450° F. Draw down ratiosrange from 20-50, or around 30-40.

[0052] We contemplate that certain extrusion conditions may be used inan effective amount, in combination or singly, to achieve one or more ofthe physical properties discussed herein. By-effective amount we intendthat with the guidance given herein, coupled with ordinary skill in theextrusion art, the skilled person could select conditions to so achievethese properties.

[0053] Definitions and Testing Protocols Melt Index (MI) g/10 min. ASTMD-1238, condition E@ 190° C. Density g/cc ASTM-D-1238 Dart Drop ImpactF₅₀ g and g/mil ASTM D-1709 Elmendorf Tear g (g/mil) ASTM D-1922 SecantModulus (1%) psi ASTM D-790A Shrinkage % Univation Test Procedure* #indicates expansion of a dimension after heating, compared to its preheating dimension

[0054] Melt strength was measured at 190° C. using a commercialinstrument (Rheotester 1000) from Goettfert, Germany.

[0055] CRYSTAF data was obtained using a commercial instrument (Model200) from PolymerChar S. A., Valencia, Spain. Using a technique outlinedin Macromol. Mater.Eng. 279, 46-51 (2000).

[0056] MWD, or polydispersity, is a well-known characteristic ofpolymers. MWD is generally described as the ratio of the weight averagemolecular weight (Mw) to the number average molecular weight (Mn). Theratio of Mw/Mn can be measured by gel permeation chromatography (GPC)techniques, or indirectly, by measuring the ratio of I₂₁ to I₂ (meltindex) as described in ASTM D-1238-F and ASTM D-1238-E respectively.

EXAMPLES Example 1

[0057] Catalyst Preparation

[0058] bis(n-propylcyclopentadienyl)hafnium dichloride metallocene wassynthesized according to procedures well-known in the art.Methylaluminoxane (MAO) 1140 cc of a 30 wt % solution in toluene(obtained from Albemarle Corporation, Baton Rouge, La.) was added to aclean, dry 2 gallon vessel and stirred at 60 rpm and 80° F. for 15 min.An additional 1500 cc of toluene is added with stirring. The metallocene(21.6 g) was dissolved in 250 cc toluene and the transfer vessel wasrinsed with an additional 150 cc of toluene. The metallocene/MAO mixturewas stirred at 120 rpm for 1 hour. Next, 850 g of silica, Davison 948(W. R. Grace, Davison Chemical Division, Baltimore, Md., dehydrated at600° C.) was added and stirred for 55 min. The catalyst was then driedat 155° F. for 10 hours while being stirred at 30 rpm.

Examples 2-19

[0059] Polymer Production

[0060] Examples 2-19 are polymerized using the catalyst in Example 1according to the conditions listed in Table I.

[0061] Granular resin was dry-blended with 500 ppm Irganox® (IR)(available from Ciba-Geigy) 1076, 2000 ppm IR168 and 800 ppm FX5920A(processing aid from Dynamar) using a double-cone blender. Some of theresins as shown in Table contained Eurucarnide and or ABT 2500 as slipand anti-block respectively. A white masterbatch was also added to someexamples a mixture of 50% TiO2 in a polyethylene carrier. Pelletizing ofthe resins was carried out on a Farrel continuous mixer (4LMSD) using aspecific energy input of 0.125 hp-h/lb. Output rate was 500 lb/h andmelt temperature was 226° C.

[0062] Film Production

[0063] Examples 2-19 and comparative examples 20-22 were extruded intofilm using a 2.5″ Battenfield Gloucester blown film line (30:1 L:D)equipped with a 6″ oscillating die and a Future Design® air ring. Outputrate was 188 lb./hour (10 lb./hour/inch die circumference). A standard“hump” temperature profile is used (where BZ is barrel zone and thetemperatures are in ° F.):

[0064] BZ1=310/BZ2=400/BZ3=380/BZ4=350 /BZ5=350/Adapter=390/Die=390° F.

Example 23

[0065] Catalyst Preparation

[0066] bis(n-propylcyclopentadienyl)hafnium dichloride metallocene wassynthesized according to procedures well-known in the art.Methylaluminoxane on silica (MOS) was obtained from UnivationTechnologies commercial catalyst facility at Mt. Belvieu, Tex.

[0067] In a glove box, 704 grams of MOS (MAO on Silica) was measuredinto a 3 L beaker. 3486 grams of de-gassed Witco Kaydol® mineral oil wasadded to the MOS and mixed with a Braun hand mixer until evenlydispersed (˜5 minutes). Then 12.05 grams of bis(n-propylcyclopentadienyl) hafnium dichloride metallocene was added andmixed for another 10 minutes. The catalyst was formulated at a 118:1Al:Hf molar ratio and 0.65 wt % hafnium transition metal in the solids.The final slurry concentration was 17 wt % solids in oil. Themetallocene/MOS/oil slurry was transferred from a beaker to the catalystlab mix tank via a 2 liter Hoke bomb. The metallocene/MOS/oil slurry wasmixed for approximately 19 hours in the mix tank, then the slurry wasoff-loaded to a 2 L bomb.

[0068] The mineral oil was de-gassed in 3-gallon batches. The 3-gallon,round bottom flask consists of an agitator, a sintered glass spargingtube, a vacuum source, and a heat source. Vacuum was pulled on the flaskwhile the oil was sparged with high purity nitrogen. During this time,the oil was heated to 110° C. The entire process lasts for 8-14 hours.

Examples 24-26

[0069] Polymer Production

[0070] Examples 24-26 were polymerized in a nominal 14″ diameterfluidized bed reactor using the catalyst in Example 23 according to theconditions listed in Table I. The slurry catalyst was transferred fromthe 2 liter Hoke bomb into an inerted agitated reservoir. The materialwas fed to the reactor using a syringe feeder through a {fraction(3/16)}″ injection tube. A 4 lb./hr nitrogen feed to the injection tubefacilitated proper catalyst dispersion into the reactor. Reactor gascomposition was monitored by GC and maintained by adjusting monomer andinert feed rates and reactor vent as needed. Resin was discharged fromthe reactor to a fiberpack in batch-wise fashion to maintain a bedweight of approximately 110 lbs. A wet nitrogen purge was fed to thefiberpack at approximately 5-10 lb/hr.

[0071] The granular resin was dry-blended with 500 ppm IR1076, 2000 ppmIR168 and 800 ppm FX5920A using a double-cone blender. Pelletizing ofthe resins was carried out on a Werner & Pfleiderer ZSK 57 mm twin screwextruder. Output rate was 150 lb/h and melt temperature ranged from205-219 ° C. depending on melt index of the resin. Pellet count was34-36 pellets/gram.

[0072] Film Production

[0073] Examples 24-26 and comparative examples 27-28 were extruded intofilm using a 2.5″ Battenfield Gloucester blown film line (30:1 L:D)equipped with a 6″ oscillating die and a Future Design air ring. Outputrate was 188 lb/h (10 lb/h/in die circumference). A standard “hump”temperature profile was used:BZ1=310/BZ2=400/BZ3=380/BZ4=350/BZ5=350/Adapter=390/Die=390° F.

[0074] Film Properties

[0075] Film properties are shown in Table II for Examples 2-19 andComparative Examples 20-22. The Comparative Examples are metallocenecatalyst produced polyethylenes, Example 20 is Exceed® 1018 CA andComparative Example 21 is Exceed ECD 313. Comparative Example 22 is acommercial Z-N LLDPE, NTX 0-95, all available from ExxonMobil ChemicalCompany. Table clearly shows that for Examples 2-19, the MD tear/TD tearratios are all greater than 0.9, with an absolute MD tear value of atleast 350 g/mil.

[0076] Similarly, in Table III, Examples 24-26, the MD/TD ratios are allabove 1.0, with dart values in excess of 500 g/mil, in contrast to theComparative Examples 27-28. Comparative Example 27 is a commerciallyavailable mLLDPE available from ExxonMobil Chemical Company (same gradeas Comparative Example 20 ). Comparative Example 28 is a commerciallyavailable mLLDPE available from Dow Chemical Company, Elite® 5400. TABLEI Examples 2-19 24 25 26 Production Rate (lb/h) 150 27 31 40 Hydrogen(ppm) 293 311 300 301 C2 partial pressure (psia) 252 234 233 240 C6/C2concentration ratio 0.015 0.021 0.023 0.022 Temperature (deg C.) 76.776.3 76.3 76.3 Residence Time (h) 4.1 3.9 3.4 2.7

[0077] TABLE II Ex. 2 #3 Ex. #4 Ex. #5 Ex. #6 Ex. #7 Ex. #8 Ex. #9 DieGap 60 60 60 60 60 45 90 60 Melt Temperature (° F.) 396 397 432 396 396396 395 392 Inner Die Temperature 395 394 423 390 390 396 391 390 (° F.)Output (lb/h) 189 190 191 188 188 189 189 151 BUR 2.5 2.5 2.5 2.0 3.02.5 2.5 2.5 FLH 18 21 29 21 18 21 19 14 Comments CHARACTERIZATION MI(I2) 0.79 HLMI (I21) 22.9 MIR (I21/I2) 29.0 Resin Density (g/cc) 0.9195Melt Strength (cN) 5.4 1% Secant Modulus (psi) SEC_MOD_MD 28,280 29,37028,590 28,900 27,460 28,140 29,280 28,170 SEC_MOD_TD 36,210 36,54035,080 40,620 33,030 36,590 36,460 35,320 Elmendorf Tear ELM_TEAR_MD 730750 350 900 450 610 730 550 (g/mil) ELM_TEAR_TD 590 600 540 760 510 580650 580 (g/mil) MD/TD Tear 1.24 1.25 0.65 1.18 0.88 1.05 1.12 0.95 DartDrop (Method A) (g) 520 420 680 170 690 610 250 470 (570, 300) (650)(g/mil) 710 550 910 230 1030 830 330 620 (760, 400) (880) Gauge Mic(mils) 0.73 0.75 0.74 0.72 0.67 0.73 0.74 0.75 Shrink (%) MD 75 74 68 7974 74 76 73 TD −20 −24 −21 −45 −9 −24 −27 −17 Ex. #10 Ex. #11 Ex. #12Ex. #13 Ex. #14 Ex. #15 Ex. #16 Die Gap 60 60 60 60 60 60 60 MeltTemperature (° F.) 400 397 397 397 396 396 396 Inner Die Temperature 391402 400 399 392 396 397 (° F.) Output (lb/h) 225 187 191 188 187 190 190BUR 2.5 2.5 2.5 2.5 2.5 2.5 2.5 FLH 23 19 19 19 19 19 19 Comments 900ppm slip 900 ppm slip 900 ppm 900 ppm slip 900 ppm slip 4000 ppm 4000ppm slip 2500 ppm 4000 ppm ABT2500 ABT2500 no AB ABT2500 ABT2500 3%white MB CHARACTERIZATION MI (I2) 0.79 HLMI (I21) 22.9 MIR (I21/I2) 29.0Resin Density (g/cc) 0.9196 Melt Strength (cN) 5.6 1% Secant Modulus(psi) SEC_MOD_MD 28,960 28,660 28,130 27,570 27,860 25,890 26,170SEC_MOD_TD 36,740 36,870 33,510 34,560 33,740 29,640 29,683 ElmendorfTear ELM_TEAR_MD 800 700 640 610 720 660 710 (g/mil) ELM_TEAR_TD 610 640660 610 710 730 (g/mil) MD/TD Tear 1.31 1.00 0.92 1.18 0.93 0.97 DartDrop (Method A) (g) 350 350 410 360 330 (380) 540 470 (g/mil) 490 460540 480 460 700 590 (520) Gauge Mic (mils) 0.72 0.75 0.76 0.75 0.73 0.780.8 Shrink (%) MD 74 75 76 76 71 74 75 TD −26 −28 27 −23 −23 −25 −25 Ex.#17 Ex. #18 Ex. #19 Cmp. #20 Cmp. #21 Cmp. #22 Die Gap 60 60 60 60 60 60Melt Temperature (° F.) 396 396 395 401 397 432 Inner Die Temperature398 398 397 406 400 422 (° F.) Output (lb/h) 188 188 188 190 188 186 BUR2.5 2.5 2.5 2.5 2.5 2.5 FLH 19 19 19 24 24 25 Comments 3% white 3% white900 ppm slip no slip 450 ppm 1400 ppm MB MB 5000 ppm no AB slip slip 900ppm slip 900 ppm slip ABT2500 4500 ppm 8000 ppm 4000 ppm 4000 ppm AB ABABT2500 ABT2500 CHARACTERIZATION MI (I2) 0.96 1.16 1.00 HLMI (I21) 15.618.7 25.5 MIR (I21/I2) 16.3 16.1 25.5 Resin Density (g/cc) 0.9197 0.92180.9226 Melt Strength (cN) 3.7 3.7 4.6 1% Secant Modulus (psi) SEC_MOD_MD26,040 26,500 26,810 24740 22290 23510 SEC_MOD_TD 30,070 30,310 29,67027450 24210 25570 Elmendorf Tear ELM_TEAR_MD 680 670 650 310 250 400(g/mil) ELM_TEAR_TD 690 830 710 500 420 760 (g/mil) MD/TD Tear 0.99 0.810.92 0.62 0.60 0.53 Dart Drop (Method A) (g) 320 270 520 390 490 350(g/mil) 430 450 660 530 630 460 Gauge Mic (mils) 0.74 0.59 0.79 0.730.78 0.77 Shrink (%) MD 76 78 74 56 54 62 TD −24 −25 −23 −14 −11 −16

[0078] TABLE III Example #24 Example #25 Example #26 Comp. Ex. #27 Comp.Ex. #28 Die Gap 60 60 60 60 60 Melt Temperature (° F.) 400 403 403 403397 Inner Die Temperature (° F.) 402 406 394 401 396 Output (lb/h) 190188 190 192 188 BUR 2.5 2.5 2.5 2.5 2.5 FLH 20 20 23 25 22CHARACTERIZATION MI (I2) 0.82 0.6 0.97 1.04 1.03 HLMI (I21) 23.2 18.1629.4 17.2 29.8 MIR (I21/I2) 28.3 30.3 30.3 16.5 28.9 Melt Strength (cN)5.5 6.8 5.4 Resin Density (g/cc) 0.9194 0.9165 0.9201 0.9184 0.9169 1%Secant Modulus (psi) SEC_MOD_MD 31130 26740 31180 24920 26240 SEC_MOD_TD38180 30130 39510 28010 29550 Intrinsic Tear (g/mil) 320 310 340 350 460MD Tear/Intrinsic Tear 2.00 2.0-2.2 1.80 0.80 1.00 Elmendorf TearELM_TEAR_MD (g/mil) 640 670 (630) 600 280 450 ELM_TEAR_TD (g/mil) 610560 580 460 680 MD/TD ratio 1.05 1.13-1.20 1.03 0.61 0.66 Dart Drop(Method A) (g) 480 650 520 480 460 (g/mil) 640 890 690 650 620 Gauge Mic(mils) 0.75 0.75 0.75 0.74 0.74 Shrink (%) MD 71 78 67 55 77 TD −20 −22−19 −4 −26

[0079] TABLE IV Example #24 Example #25 Example #26 Comp. Ex. #27 Comp.Ex. #28 CHARACTERIZATION MI (I2) 0.82 0.6 0.97 1.04 1.03 MIR (I21/I2)28.3 30.3 30.3 16.5 28.9 Melt Strength (cN) 5.5 6.8 5.4 Resin Density(g/cc) 0.9194 0.9165 0.9201 0.9184 0.9169 PROCESS DATA Output (lb/h) 190188 190 192 188 ESO (lb/HP-h) 10.96 10.32 11.79 10.44 11.92 HeadPressure (psi) 3710 4160 3470 3820 3340 Die Pressure (psi) 2540 29002320 2520 2240 Motor Load (amps) 68.3 71.4 64.0 72.4 63.1 Inner DieTemperature (° F.) 402 406 394 401 396 Melt Temperature (° F.) 400 403403 403 397 Screw Speed (rpm) 59.7 59.9 59.7 59.7 58.9 Line Speed (fpm)235 233 232 232 229 Gauge (mils) 0.75 0.75 0.75 .76 0.75 FLH (in) 20 2023 25 22 Air (%) 76.7 77.9 63.0 63.0 63.2

[0080] Although the present invention has been described in considerabledetail with reference to certain embodiments thereof, other embodimentsare possible. For example, while blown films with an improved machinedirection Elmendorf tear is exemplified, other properties and filmimprovements are contemplated. Therefore, the spirit and scope of theappended claims should not be limited to the description of theembodiments contained herein.

We claim:
 1. A film comprising a polyethylene possessing a density inthe range from 0.910 to 0.940 g/cc, a M_(w)/M_(n) value of 2.5 to 7 anda CDBI value of ≦55%; the film characterized in having an MD tear to TDtear ratio of ≧0.9 for a film of ≦1 mil.
 2. The film of claim 1, furthercharacterized in that the MD tear of the film increases with increasingMD shrinkage.
 3. The film of claims 2, wherein the MD tear is ≧500 g/miland the MD shrinkage is ≧70%.
 4. The film of claim 1, wherein said filmhas an MD tear ≧400 g/mil.
 5. The film of claim 1, dart drop impact ≧500g/mil.
 6. The film of claim 1, made in a gas phase process by contactingethylene, at least one comonomer having from 4 to 8 carbon atoms and ahafnium transition metal metallocene-type catalyst system.
 7. The filmof claim 6, wherein the hafnium transition metal metallocene-typecatalyst system is in a mineral oil slurry when contacted with ethyleneand comonomer.
 8. The film of claim 6, wherein the hafnium transitionmetal metallocene-type catalyst system is the product of the combinationof a hafnium transition metal metallocene-type catalyst and apresupported activator in oil or a hydrocarbon.
 9. The film of claim 6,wherein the hafnium transition metal metallocene-type catalyst system isdry when contacted with ethylene and comonomer.
 10. The film of claim 1,wherein the film is obtained by extrusion at a draw down ratio of from20 to 50, a die gap of from 30 to 120 mils, and at from ≧8 lbs ofpolymer per hour per inch of die circumference.
 11. The film of claim 1,wherein the polyethylene possesses a melt index of from 0.1 to 10dg/min.
 12. The film of claim 11, wherein the polyethylene possesses anI₂₁/I₂ of from ≦40.
 13. The film of claim 1, further possessing ≦5 ppmtitanium.
 14. The film of claim 1, wherein the polyethylene of density0.915 to 0.940 g/cc possesses ethylene derived units, and 1 to 5 mole %comonomer derived units.
 15. The film of claim 6, wherein the contactingis in a single gas phase reactor to form the polyethylene, thepolyethylene thus extruded to form the film.